When we think of bone marrow, we usually think of it as being red in color, but yellow bone marrow exists as well, and exists for a very important reason. We don't hear much about yellow marrow, so one might initially think of yellow marrow as being red marrow that is diseased or otherwise infected. Such is not the case however. Yellow marrow performs a couple of very important functions.

Bone Marrow And Stem Cells - Bone marrow lies in the interior portion of our larger bones. Unlike the outer surface of the bone, which is hard and relatively inflexible, the bone marrow is located in the inner part of the bone which is flexible and somewhat porous. The bone marrow itself could be described as being a soft gelatinous tissue. Bone marrow can therefore be aspirated, or drawn out through a hollow needle, when a biopsy is needed, or the marrow needs to be collected for possible donor use. Bone marrow contains two types of stem cells, stem cells which produce red blood cells, and stem cells which produce fat, bone, and cartilage. The stem cells that produce red blood cells are called hematopoietic stem cells. The hematopoietic stem cells also produce white blood cells and platelets. The stem cells found in the yellow marrow, produce fat, bone, and cartilage, and are called stromal cells.

Why Some Bone Marrow Is Yellow - At birth, all of our bone marrow is red. As we get older, some of the bone marrow turns yellow, and by the time we are adults nearly half of our bone marrow is yellow. As we approach old age, more than half of our bone marrow is yellow. Yellow marrow contains a very high percentage of fat cells, which accounts for the yellow color.

Although confined to the interior portions of our long bones, bone marrow makes up approximately 4% of our total body weight. Although the percentage of marrow that becomes yellow increases with age, if there is a shortage of red blood cells in the body, yellow bone marrow can quickly convert back to red bone marrow, and the percentage can change, and can literally change overnight. In fact, yellow bone marrow can be converted into red bone marrow in about an hour. While the predominate colors of bone marrow are red and yellow, our body contains a certain amount of pink bone marrow as well. While red one marrow is concentrated at the ends of the long bones, pink bone marrow exists in the middle portion of the same bones.

Bone Marrow Diseases - The two diseases which are most likely to affect either red or yellow bone marrow are tuberculosis and cancer. It is normally the red marrow cells that tend to be affected, where malignancies, brought about by various forms of leukemia, inhibit the production of red blood cells. Chemotherapy and radiation therapy, used to fight leukemia and other types of cancer also present a danger to the bone marrow cells. When bone marrow cells are destroyed, it can weaken the immune system, as the bone marrow is a part of the lymphatic system, and thus is also a part of the immune system.

Although yellow bone marrow is instrumental in producing certain types of cells, its primary function is to store fat. If we are on the verge of starvation, and are turning into skin and bones, the last remaining source of energy in our body will be the fat stored in yellow marrow. Once that fat is gone, there is no more.

Since the yellow marrow primarily stores fat, and is not an active contributor in the production of red blood cells, most of the diseases affecting bone marrow affect the red bone marrow. The yellow marrow is usually affected only secondarily, as it may need to transform itself into red bone marrow if the production of red blood cells is being inhibited by disease. A malignancy can of course affect the cells in the yellow marrow as well as those that make up red marrow, and yellow marrow is just as sensitive to radiation as red marrow is. Still, when it comes to treating diseases that can affect bone marrow, it's the red marrow that is usually of greatest interest or concern.

Bone Marrow Transplants - As far as bone marrow transplants are concerned, it's only the red marrow that is of interest, as the goal is normally one of removing healthy, red blood cell producing stem cells from the marrow of the donor and transferring them to the recipient. In medical circles, the term “hematopoietic stem cell transplantation” is slowly replacing the term “bone marrow transplantation”. Hematopoietic stem cell transplantation is a much more accurate description of what is actually being one. Little would be gained by transplanting yellow marrow cells from one person to another. Not all bone marrow transplants are allogeneic transplants, which involve a donor and a recipient. Bone marrow can be withdrawn from a patient, and after the patient has received a certain type of treatment, can be re-injected. This is called an autologous transplant.